The Phase of the Moon

Will moonlight interfere with nighttime observing when you plan to take vacation next summer? What was the Moon's exact phase on the day you were born? When's the next opportunity to photograph a young crescent Moon?

These and other questions are easy to answer with SkyandTelescope.com's Moon Box, a JavaScript utility that will open in a new browser window. It accurately shows the Moon's appearance on any date from 4000 BC to AD 8000, a whopping span of 120 centuries!

When you launch it, the animated Moon lets you know it is waiting for your input. Simply click the Calculate button to find the phase of the Moon right now. The animation stops at the Moon's current appearance, and the phase is described in words at the bottom.

To get the Moon's phase on any other date, use your mouse to select the month and day from their list boxes at top. If the year needs to be changed, click within its box and retype the numbers. Be sure the kind of year, AD or BC, is properly selected, and click the Calculate button again... www.skyandtelescope.com

Size & Distance

The moon, Earth's only natural satellite, is large as moons go. It is fifth in diameter among planetary satellites, more than two-thirds as large as Mercury, and more than three times the diameter of the largest asteroid. It is, in fact, over one-fourth the size of the earth, with a diameter of 2160 miles (3476 kilometers). Since the moon is a relatively near neighbor, we can measure its distance easily by geometrical methods. The average is 238,857 miles (384,403 kilometers).

Brightness

Next to the sun, the full moon is the brightest object in the heavens. However, its surface is rough and brownish and reflects light very poorly. In fact, the moon is about the poorest reflector in the solar system. The amount of light reflected by a celestial object is called the albedo (Latin: albus, white). The moon relects only 7% of the sunlight that falls upon it, so the albedo is 0.07.

The Far Side

People often refer to "the dark side of the moon", but there is no such thing. The sun shines on all sides of it in turn. However, there is a "far side of the moon" which is never seen from the earth. Over the eons, the gravitational forces of the earth have slowed down the moon's rotation about its axis until the rotational period exactly matches the revolution period about the earth. You can see this effect by using two round objects such as softballs. Hold one of the balls stationary, to represent the earth. Now move the other ball around the "earth" without twisting your wrist. You will see that people on the "earth" would see all sides of the "moon". However, if you slowly spin the "moon" on its trip around the "earth", you will see that you can time it so only one side of the "moon" is ever seen from the "earth". That's why the features you see on the face of the moon never change.